1. Field of the Invention
This invention relates to a method of applying a coating solution to a web which is continuously run and to apparatus for practicing the method, more particularly, to a coating method and a coating apparatus in which the coated film is effectively smoothed.
2. Description of the Prior Art
The term "web" as used herein is not limited and is intended to mean a relatively long, flexible, belt-shaped support such as: a plastic film of cellulose triacetate, polyamide, polyimide, polycarbonate, polyethylene terephthalate or polyvinyl chloride; paper; synthetic paper; a metal foil or aluminum or copper; a sheet of glass or ceramic and the like.
The term "coating solution" is not limited and is intended to mean a variety of coating solutions different in composition depending on the purpose of use. Examples of "coating solutions" include coating solutions for forming a photosensitive emulsion layer, a base layer, a protective layer and a back layer as are conventionally used in photographic photosensitive materials; coating solutions for forming a magnetic layer, a base layer, a wetting layer, a protective layer and a back layer as conventionally used in magnetic recording mediums; and coating solutions for forming an adhesive layer, a coloring layer, a rust-proofing layer, etc. Such coating solutions are typically made up of essential components, a binder, and, if desired or necessary, water or an organic solvent or can be a dispersion containing various additives.
In order to form a coated film on the above-described webs to a desired thickness, the following methods are not extensively employed: a dip, reverse roll, gravure roll, extrusion hopper or slide hopper is used to apply an excess of coating solution to the web which is run continuously. Then, metering means such as an air knife, blade or coil bar (or wire bar) is applied to the coated layer to scrape off surplus coating solution to coat the web with the coating solution to the desired degree or thickness.
In coating a web with a coating solution of high viscosity, such as a magnetic solution for forming a magnetic recording medium, a coil bar has been extensively employed as the metering means because it is simple in structure and can be readily handled as compared to an air knife or blade, and the coated surface is relatively stable in quality.
A conventional coating method using a coil bar is as shown in FIG. 1.
As shown in FIG. 1, web 1 is continuously run in the direction of the arrow and a coating solution 3 is applied to the lower surface of the web 1 to a thickness larger than finally required to form coated film 5 in the liquid state. Surplus coating solution 7 is scraped off by coil bar 6 so that coated film 8 is formed which is regulated by the gap which is defined by the cylindrical wall of the coil bar 6 and the surface of the web 1. If necessary, for instance in the case of forming a magnetic layer, the resulting product is subjected to magnetic field orientation and drying, and is then wound. In FIG. 1, reference numeral 4 designates guide rolls.
Coil bar 6, as shown in FIG. 2, is made up of a core material or a rod member 21 and a wire 22 which is wound in the form of a single coil on the cylindrical wall of the rod member 21 with the turns being in close contact with each another. In general, rod member 21 is a stainless steel, iron or brass rod 1 to 3 mm in diameter and the wire 22 is a stainless steel, trifluoroethylene homopolymer, tetrafluoroethylene homopolymer, or tetrafluoroethylene-hexafluoropropylene copolymer wire 0.04 to 0.05 mm in diameter.
In addition to the above-described coil bar, a small coil bar less than 4 mm in maximum coil radius which was proposed by the present inventors may also be employed (see Japanese Patent Application No. 41060/1981).
Depending on the purpose of use of the product, it is required to smooth the surface of the coated film or coated layer. The coated film or coated layer is smoothed by suitable means after being subjected to metering with the coil bar.
In a conventional coating method as shown in FIG. 1, the web is left in a gaseous circumference during the time interval from the instant that the coating solution is applied to the web until the metering operation is carried out with the coil bar 6 or during the time which from the instant that the metering operation is effected with the coil bar until the smoothing operation is carried out with a smoother (not shown). During such time interval the coated film is not dried or hardened, and, therefore, the metering or smoothing operation is liable to be affected by the viscosity of the coating solution, interfacial tension, and web velocity.
During metering or smoothing, air may go between the metering member and the coated film or the coating material which has been solidified on the metering bar may mix therewith, thus harming the coated surface. This tendency becomes significant as the viscosity of the coating solution increases and coating velocity increases. Therefore, rib-like (streak-like) stripes are formed on the coated surface degrading the smoothness of the surface.